That switch was a boostas you said. They were bypassed because the boost circuit sounded very bad. The pot was in some amps were completely bypassed so there was no bleed through the circuit. Great video.
Great video Stuart.. Seen many of these silver face twins worked on. They all seem slightly different but present with similar issues that require inventive repair solutions and careful searches for the correct schematics ! At least you get a good physical work-out humping them around workbench ..
This video demonstrates to me the clear drop in quality of the CBS years. They used to be beautiful works of art, but by this time it became a rats nest of tangled wires and sloppy looking boards. The silverface amps I’ve played and owned all sounded good, however. But then something goes awry and you have this! Great vid, extremely good work!
Hi! Lovely vid as usual. Wondering if I could ask - what is the purpose of the caps which go with the diodes in the rectifier? Filtering? Thought that was done by the big filter caps?
Patrick, the small caps help to prevent the diodes from failing due to incoming voltage surges, and from the inductive voltage-spike that occurs when the transformer is energized (transformer coils are essentially big inductor coils, and hitting them with turn on voltage causes a back-EMF spike). The filter caps themselves will also draw a significant spike of current as they charge up, all of which can conspire to blow diodes or cause fuses to fail for no apparent reason.
Hi Gary I buy from all over the place but I try ti use eBay as much as possible as it helps out the small traders there. Most things you can get on eBay!
Hi, thanks for the video. As i am in the process of learning to service a tube amp and i had the twin 100 watt here in my place. Do you mind share what device were you using to check the capacitor for leakage? Thanks
Hi I'm pleased you are anjoying the videos. The device you are afteer is called an 'insulation resistance tester' and you can buy one on eBay quite cheaply. Best Regards
Ur welcome Mr Stuart, and thanks for the info regarding the device. Another question if you don’t mind answer it, from your experience, what a good brand for variac? And not to expensive for me to starting this amp repairing journey. Thanks!
@@stuartukguitarampguy5830 , tune your guitar to an open chord and you can probably fake your way through a few half-decent riffs; Keith Richards is fond of open G tuning! (DGDGBD, low to high). Or, at the very least, it won't sound discordant when you strum the open strings just to see if the amp is passing signal as you work on it.
Ooh, I had a few of these on the bench recently, mostly circa 1980s vintage and all as tatty as the one in the video. I couldn't figure out why the SF MV tend to look so much worse than a lot of older BF non-MV amps.
Less than 12 minutes in, and I'm jumping in prematurely to comment (standard operating procedure for me!😁 ---- otherwise I'll forget my train of thought): so, does this amp have or did it have the pull-boost circuit found in some 70's Fenders? That green module hanging there (with what looks like original wiring) might have been a switch mounted to the back of a pot, which activated a boost circuit when you pulled the knob out. Most players don't like the boost circuit, so....
Well spotted. You'll have watched the rest of the video by now and see that I eventually discovered this. SWhen I put it all back together it didn;t really do anything. I'm not TOTALLY convined I got that switch diagram right tbh. If it comes back in I may give that another go.
@@stuartukguitarampguy5830 , you never really showed the front face of the amp, but it should be marked "Pull Boost" on the front panel, right next to the pot in question! That would tend to be a clue.....
@@stuartukguitarampguy5830 , perhaps the original knob itself said, or would have been marked, "pull boost". Maybe the original pot and knob were broken while loading the amp in and out of the van or the club, and that's why they had to be replaced.....its been so long since I saw a Fender with the boost pot that I can't quite remember what they looked like. If it was an early production amp with this new feature, it might have left the factory with an earlier type face plate, as the bean counters typically insisted on using up all the old parts.
@@goodun2974 Yes the pot had defintely been changed. I think they couldn;t be bothered to get the switched version, probably didn't even use the push/pull, so decided to just put a standard pot in there.
Don't forget that Brimar are now supplying valves again branded as Brimar. Not many know this!! I did away with my master volume. The amp sounds more open with master at full. There is no reason for master vol as amp is clean from 1 to 7. If you can run it at 7 then you are in a stadium!!
Hi Richard. Do you have any more details? Where are they located? 99% of all valves are produced either by JJ or an American company which owns manys brands like Sovtek etc. The main Chinese factory burned down 2 years ago and has not been rebuilt. There is currently a massive worlwide shortage of valves as a result.
Hi Great video...I waited and waited to just how you wired up the new master volume pot!..Then you didn't show me how to do it "Im sad"...My original master volume pot has a dead spot between 7 and 9 so I need to replace it but really don't know how?...Shame you didn't show that cause there are no other videos anywhere to show that..but I sat right through the video...if you could possibly show just how you did it everyone would want to watch...cheers Frank "The Memphis Suns" Australia ...Originally from Bedfordshire Midlands!
Hi Frank. Oh, shame. Slrry it';s been a while now and I can;t remember. On your amp, can you not just litreally replace the volume control making note of what goes where?
@@stuartukguitarampguy5830 Yes but these original push pull knobs have many lugs..I'm a novice so really don't know what goes where?..But I found CTS make a push pull 1 meg pot I may buy one of those see how I go!..I have two vintage Fenders a mint 78 Pro Reverb 70 watts and a 100 watt 76 Twin ...The mint Pro Reverbs master pot cuts out at around 7 then activates back aroynd 9 to 10...still works but of course not totally original working condition hence my struggle to get it fixed.. Checkout my original Rockabilly band THE MEMPHIS SUNS..we're popular there in the UK and USA..
Is that a bridge rectifier arrangement? Older Fenders used a full-wave rectifier circuit, but those transformers needed a B+ center-tap; CBS probably bought cheaper transformers without a C-T. Also, how does that Megger generate 500 volts? Built-in switching supply? (Seems unlikely). Or does it run off of AC wall voltage? The old-school telephone-system installers I used to work with used a Megger occasionally, but those ran off of an oddball battery, I think it was either 45 volts or 90 volts, I can't quite remember.
On the Megga, I think it's simply a step-up transformer and a transistor which shorts the primary to ground at a frequency of 20KhZ or whatever. Bit likew the old shocking coil arrangement. You dont need any current of course.
@@stuartukguitarampguy5830 , is the Megger powered by mains AC, or a battery? The original, old fashioned way to step up a DC voltage was to use a "mechanical" vibrator that chopped the DC up into fast pulses which could then be run through a step-up transformer. This is how valve-type car radios of the 1940s and 50's developed the necessary high voltages, until so called "space charge" tubes were developed that could work efficiently on plate voltages as low as 12 V.
